Taxol, a member of the class of compounds known as taxines, comes from the bark of the Pacific yew tree, Taxus brevifolia, and has been found useful in the treatment of various cancers. For example, taxol has been used in treating ovarian, breast, non-small cell lung, and head and neck carcinomas. David M. Peereboom et al., "Successful Re-Treatment with Taxol After Major Hypersensitivity Reactions", Journal of Clinical Oncology, 11 (5), pp. 885-890 (1993). One of the difficulties in administering taxol is that the drug is insoluble in water. The present state of the art in taxol formulation requires a 50:50 mixture of Cremophor-EL surfactant (polyoxyethylated castor oil) and ethanol in order to solubilize the drug. Unfortunately, this taxol formulation leads to a relatively high incidence of major hypersensitivity reactions (HSRs) upon intravenous administration. These HSRs have been attributed to the unusually high concentration of Cremophor-EL required to solubilize the taxol. Id.
There have been other attempts to provide a taxol formulation, the most successful of which has been incorporation of the drug into a liposomal formulation. However, this preparation suffers from the fact that it is difficult to achieve a quantitative incorporation of the drug into the liposomal compartment. Furthermore, the product must be lyophilized and stored as a powder because taxol precipitates from aqueous liposomal formulations within a week of storage. For this reason, the liposomal formulation must be freeze dried and reconstituted prior to use.
Attempts to formulate taxol in a stable lipid emulsion have been unsuccessful. Taxol is reported to be insoluble in lipid emulsions such as Intralipid.RTM., which contains soybean oil, or Liposyn.RTM., which contains a mixture of soybean and safflower oils. L. C. Collins-Gold et al., "Parenteral Emulsions for Drug Delivery", Advanced Drug Delivery Reviews, 5, pp. 189-208 (1990). Heating taxol in either soybean oil or safflower oil, even upon sonication, does not result in the dissolution of appreciable amounts of taxol, and addition of taxol to a lipid emulsion during the homogenization step meets with equally disappointing results. Emulsions incorporating up to 15 mg/ml of taxol have been formulated with triacetin, L-.alpha.-lecithin, Polysorbate 80, Pluronic F-68, ethyloleate and glycerol. However, these emulsions are highly toxic and unstable. B. Tarr et al., "A New Parenteral Emulsion for the Administration of Taxol", Pharmaceutical Research, 4, pp. 162-165 (1987).
Therefore, there is a clear need for a stable, easily prepared, biocompatible, efficacious formulation of a taxine such as taxol exhibiting minimal side effects.